Conventionally, an RF field is applied to an optical device in the region of a waveguide so as to change the refractive index of the waveguide to bring about modulation in intensity and/or phase of any light being carried within that waveguide.
The modulating RF electric field is usually applied via an electrode formed on a buffer layer covering a substrate surface on which the waveguide has been prepared. The buffer layer typically comprises a dielectric film that has a refractive index that is smaller than that of the waveguide. One of the functions of the buffer layer is to reduce optical losses that are due to absorption within the metallic electrodes. Another function of the buffer layer is to obtain velocity matching between the microwave (RF) propagating within the electrode and the light propagating within the waveguide.
Examples of optical waveguide devices that use an electro-optical crystal substrate, such as, for example lithium niobate (LiNbO3), are Mach-Zehnder interferometric modulators, electro-optical polarisation controllers, variable optical attenuators and electro-optic switches.
Within, for example, the Mach-Zehnder modulator the relative optical path lengths of two signals derived from a single optical signal are varied as a result of refractive index changes in corresponding waveguides in response to an applied RF field to produce, upon recombination, either constructive or destructive interference according to the relative phases of the two optical signals.
A state of a preferred phase difference, which is changeable with temperature and time, is controlled by a simultaneous application of a DC bias voltage via an appropriate electrode structure. It is well known within the art that a DC bias may be required to ensure stable operation of the device even in an ordinary service atmosphere. Accordingly, many optical devices employ DC bias voltage feedback control circuits in an attempt to maintain a stable optical output signal.
The variation in the DC bias voltage as a result of the DC bias voltage feedback control is known within the art as the DC drift of the device. Indeed, a quality specification imposed upon such devices is that the applied DC bias should remain within predetermined limits over a 25-year period.
U.S. Pat. No. 5,404,412, assigned to Fujitsu Limited, discloses an optical device having reduced DC drift, that is, having an arrangement for maintaining the DC bias voltage to within predetermined limits. U.S. Pat. No. 5,404,412 addresses the confinement of DC drift by using a buffer layer that is a transparent dielectric or insulator. The layer is formed from a mixture of silicon dioxide, an oxide of at least one element selected from the group consisting of the metal elements of Groups III to VIII, Ib and IIb of the periodic table and semi-conductor elements other than silicon. For example, an embodiment of U.S. Pat. No. 5,404,412 discloses a buffer layer comprising silicon dioxide (SiO2) containing a 5 mol. % of In2O3 and a 5 mol. % of TiO2, that is, the buffer layer consists of a composition (SiO2)0 95—(TiO2)0 05 as a base that also contains a 5 mol. % of In2O3. It has been found that the DC drifts of waveguide devices having such a buffer layer are within acceptable parameters.
However, it will be appreciated that the manufacture of a buffer layer having such a complex composition is a relatively complex and expensive procedure. The accurate control of the chemical composition of such complex oxide materials in the form of a film (i.e. the buffer layer) is very difficult because of, for example, the different volatile temperatures and pressures of the source materials, and the differences in suitable film deposition conditions being depending on materials, etc.
Therefore, it is an object of the present invention at least to mitigate some of the problems associated with the prior art.